The present invention relates to a method and an apparatus for controlling the air-fuel ratio in an internal combustion engine under transient operating conditions.
There is known a closed-loop control method for controlling the air-fuel ratio, which method repeats the following process so that the air-fuel ratio in the engine will finally lie within a predetermined range. First, the running speed of the engine and the intake manifold pressure are detected. Then, a basic pulse-width of the injection signal applied to a fuel injection valve is calculated, depending upon the detected speed and pressure. This basic pulse-width of the injection signal is corrected in accordance with an air-fuel ratio correction factor, which is calculated from a detection signal produced by a concentration sensor for detecting a particular component, such as an oxygen component, in the exhaust gas. Hereinafter, this concentration sensor is referred to as an O.sub.2 sensor. In accordance with the corrected pulse-width, the actual fuel feeding rate to the engine is adjusted.
According to the above-mentioned method for controlling the air-fuel ratio, it is possible to control the air-fuel ratio so as to lie within a very narrow range in the vicinity of the stoichiometric air-fuel ratio and, hence, it is possible to maintain high levels of the functions of the three-way catalytic converter installed in the exhaust system to simultaneously remove three harmful components, such as CO, HC and NOx, contained in the exhaust gas.
In general, the air-fuel ratio can be converged within a desired range by the above-mentioned feedback control method only when the engine is under steady-state operating conditions. However, when the throttle valve is quickly operated, i.e., when the engine is under the transient operating conditions (an accelerating or decelerating condition), the air-fuel ratio after being controlled is often greatly deviated from the stoichiometric air-fuel ratio, either toward the lean side or the rich side, for a moment. This momentary change in the controlled air-fuel ratio is usually called an air-fuel ratio spike (lean spike or rich spike). During the period of acceleration, for example, the lean spike develops to a considerable degree, due to the lag in controlling the amount of fuel injection relative to the change in the amount of the intake air or due to the lag in intaking the injected fuel into the combustion chamber. The rich spike, on the other hand, takes place during the period deceleration. The air-fuel ratio spikes deteriorate the purifying functions of the three-way catalytic converter. Particularly, large lean spikes deteriorate the operation characteristics of the engine.
In order to eliminate the above-mentioned inconveniences under the conditions of the transient operation, a method has been proposed to increase or decrease the amount of fuel by a predetermined amount during the periods of acceleration or deceleration. According to this method, however, the amount of fuel is increased or decreased always by a predetermined value during the periods of acceleration or deceleration. Consequently, the increment or decrement of fuel is often deviated from an optimum value, due to variance in the quality of parts constituting the engines, due to the aging or due to the environmental changes. To determine the increment or decrement when designing the engine, a variety of values must be used to find the optimum values. With the conventional method, therefore, considerable periods of time and labor are required for designing the system.